Confusable diagnoses · PANCE / PANRE

Asthma vs Cystic Fibrosis

Written & clinically reviewed by Ryan Busch, PA-C · Updated August 2026 · Report an error

Asthma and Cystic Fibrosis are easy to mix up on the boards. Here's a side-by-side comparison — presentation, workup, imaging, and first-line treatment — drawn from our full outlines.

Asthma vs Cystic Fibrosis at a glance

  • Asthma: Chronic reversible airway inflammation with episodic bronchoconstriction.
  • Cystic Fibrosis: Autosomal recessive CFTR dysfunction causing thick secretions, lung disease, and pancreatic insufficiency.

Try two board-style questions on Asthma vs Cystic Fibrosis

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Question 1PulmonaryEasy
A 22-year-old woman with asthma presents to the emergency department with severe dyspnea. She speaks in single words, uses accessory muscles, and has a respiratory rate of 32/min, heart rate of 124/min, and SpO₂ of 89% on room air. Peak expiratory flow is 35% of predicted. After three albuterol nebulizer treatments she has minimal improvement. Which of the following is the most appropriate next step in management?
  • AIV methylprednisolone and IV magnesium sulfate
  • BOral prednisone with continued nebulized albuterol
  • CInhaled ipratropium added to continuous albuterol
  • DEndotracheal intubation and mechanical ventilation
Reveal answer & full explanation
Correct answer: A — IV methylprednisolone and IV magnesium sulfate
  • AIV methylprednisolone and IV magnesium sulfate✓
  • BOral prednisone with continued nebulized albuterol
  • CInhaled ipratropium added to continuous albuterol
  • DEndotracheal intubation and mechanical ventilation

Why IV methylprednisolone and IV magnesium sulfate is correct

  • This is a severe asthma exacerbation with life-threatening features: peak expiratory flow <40% of predicted (35%), SpO₂ <90% on room air (89%), single-word speech, accessory muscle use, and minimal response to three albuterol nebulizer treatments
  • Per current GINA guidance, a severe exacerbation not responding to initial bronchodilator therapy warrants early systemic corticosteroids plus, as an adjunct, IV magnesium sulfate 2 g over 20 minutes (smooth muscle relaxation) — given alongside (not instead of) continued bronchodilators and ipratropium, with ICU-level disposition
  • The IV route is preferred over oral dosing when severe respiratory distress makes reliable oral intake uncertain and intubation remains a possibility

Why the others are wrong

  • Oral prednisone with continued nebulized albuterol — Oral steroids with continued nebulizers suit a moderate exacerbation but are insufficient escalation for refractory, hypoxemic disease that warrants IV magnesium (premature closure on a moderate-exacerbation plan)
  • Inhaled ipratropium added to continuous albuterol — Ipratropium is a reasonable adjunct in severe asthma, but this option omits the systemic corticosteroids and IV magnesium that drive recovery; no plan without steroids is best here (right-diagnosis-wrong-step)
  • Endotracheal intubation and mechanical ventilation — Intubation is reserved for impending respiratory failure (drowsiness, silent chest, rising CO₂, exhaustion), none of which is present; maximal medical therapy comes first (anchoring on the low SpO₂)
Question 2PulmonaryMedium
A 4-year-old boy is brought in for recurrent pneumonia and chronic productive cough since infancy. His parents report bulky, greasy, foul-smelling stools and note that his skin tastes salty when they kiss him. He is at the 5th percentile for weight despite a good appetite. Exam shows mild digital clubbing, an increased anteroposterior chest diameter, and coarse crackles bilaterally. A chest radiograph reveals hyperinflation and peribronchial thickening. Which of the following is the most appropriate next diagnostic test?
  • ASweat chloride iontophoresis assay
  • BSerum immunoreactive trypsinogen level
  • CSerum immunoglobulin quantification
  • DHigh-resolution CT scan of the chest
Reveal answer & full explanation
Correct answer: A — Sweat chloride iontophoresis assay
  • ASweat chloride iontophoresis assay✓
  • BSerum immunoreactive trypsinogen level
  • CSerum immunoglobulin quantification
  • DHigh-resolution CT scan of the chest

Why Sweat chloride iontophoresis assay is correct

  • The vignette is classic cystic fibrosis: recurrent sinopulmonary infection, malabsorptive (steatorrheic) stools from pancreatic exocrine insufficiency, failure to thrive, salty-tasting skin, and clubbing — a multisystem pattern pointing to CFTR dysfunction.
  • Per CF Foundation 2017 diagnostic criteria, a quantitative pilocarpine-iontophoresis sweat chloride test is the gold-standard confirmatory study; a value >=60 mmol/L on two occasions establishes the diagnosis (30-59 intermediate, <30 normal).
  • It is the correct next step because it directly tests the underlying defect and is the most accessible confirmatory study.

Why the others are wrong

  • Serum immunoglobulin quantification screens for a humoral immunodeficiency (e.g., CVID, hypogammaglobulinemia) as a cause of recurrent infection, but it would not explain the steatorrhea, salty skin, or clubbing.
  • High-resolution CT scan of the chest would demonstrate bronchiectasis and characterize lung disease, but it confirms structural damage rather than the underlying etiology and does not establish a CF diagnosis.
  • Serum immunoreactive trypsinogen level is a newborn screening tool drawn in the first days of life; it is not validated as a diagnostic test in a symptomatic 4-year-old, in whom sweat chloride testing is indicated.
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Side-by-side comparison

FeatureAsthmaCystic Fibrosis
At a glanceChronic reversible airway inflammation with episodic bronchoconstriction.Autosomal recessive CFTR dysfunction causing thick secretions, lung disease, and pancreatic insufficiency.
Classic presentationWheezing that improves with short-acting beta-agonist; nocturnal cough; symptom relief between episodes.; Episodic wheeze, cough (often nocturnal), chest tightness, dyspnea; Triggered by allergens, exercise, cold air, viral URIs, NSAIDs, beta-blockers, emotional stress; Diurnal variation: worse at night and early morning; Expiratory…Neonatal: meconium ileus (10-20%), failure to thrive, prolonged jaundice; Pulmonary: chronic productive cough, recurrent pneumonia, sinusitis, nasal polyps, wheeze, exercise intolerance, hemoptysis, clubbing; GI: bulky greasy foul-smelling stools, malabsorption (fat-soluble vitamin deficiencies A, D, E, K), distal intestinal obstruction…
Workup / key labsGINA: history of variable respiratory symptoms + confirmed variable expiratory airflow limitation (bronchodilator reversibility, methacholine positivity, or PEF variability).; CBC (eosinophilia supports allergic phenotype); Total IgE and allergen-specific IgE if considering biologics or ABPA; FeNO (fractional exhaled nitric oxide) —…CF Foundation 2017: clinical features OR positive newborn screen OR sibling history PLUS evidence of CFTR dysfunction — sweat chloride ≥60 mmol/L (two tests) OR two disease-causing CFTR mutations OR abnormal nasal potential difference.; Sweat chloride test (pilocarpine iontophoresis) — diagnostic gold standard: ≥60 mmol/L on two…
ImagingCXR — usually normal or hyperinflation; obtain to exclude alternative diagnoses or complications (pneumothorax, pneumonia); HRCT if bronchiectasis or alternative diagnosis suspectedHRCT chest — bronchiectasis (upper lobe predominant in CF), mucus plugging, air trapping; CXR — hyperinflation, peribronchial thickening, atelectasis; Abdominal imaging if DIOS or biliary disease
First-line treatmentSince 2019, GINA no longer recommends SABA-only treatment; all adults and adolescents should receive ICS-containing therapy. The US NAEPP 2020 update differs: Step 1 = PRN SABA alone (intermittent asthma), Step 2 = daily low-dose ICS + PRN SABA, Steps 3-4 (age 5 and older) = ICS-formoterol daily and PRN (SMART); Step 1-2 (mild):…CFTR modulator therapy — disease-modifying, transformative:; Elexacaftor/tezacaftor/ivacaftor (Trikafta) — for ≥2 years with ≥1 F508del or other responsive mutation; >85% of patients now eligible; vanzacaftor/tezacaftor/deutivacaftor (Alyftrek, FDA 2024) is a once-daily triple alternative for ≥6 years with ≥1 F508del or other responsive…

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Full outlines

Asthma Cystic Fibrosis All 514 diagnoses

Educational use only. This outline is a study aid for PA students and is not medical advice or a substitute for clinical judgment. FirstPassPA is an independent study tool and is not affiliated with, endorsed by, or sponsored by NCCPA or PAEA. PANCE® and PANRE® are registered trademarks of the National Commission on Certification of Physician Assistants; End of Rotation™ is a program of the Physician Assistant Education Association.